From 311af752cfd98b534a3d5dbf30e4a202693f32dc Mon Sep 17 00:00:00 2001
From: George Claghorn <george@basecamp.com>
Date: Fri, 15 Dec 2017 10:45:00 -0500
Subject: Restrict variants to variable image blobs

---
 activestorage/app/models/active_storage/blob.rb | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

(limited to 'activestorage/app/models/active_storage/blob.rb')

diff --git a/activestorage/app/models/active_storage/blob.rb b/activestorage/app/models/active_storage/blob.rb
index acaf22fac1..fdf17ac67e 100644
--- a/activestorage/app/models/active_storage/blob.rb
+++ b/activestorage/app/models/active_storage/blob.rb
@@ -16,6 +16,7 @@ require "active_storage/analyzer/null_analyzer"
 # update a blob's metadata on a subsequent pass, but you should not update the key or change the uploaded file.
 # If you need to create a derivative or otherwise change the blob, simply create a new blob and purge the old one.
 class ActiveStorage::Blob < ActiveRecord::Base
+  class InvariableError < StandardError; end
   class UnpreviewableError < StandardError; end
   class UnrepresentableError < StandardError; end
 
@@ -110,6 +111,7 @@ class ActiveStorage::Blob < ActiveRecord::Base
     content_type.start_with?("text")
   end
 
+
   # Returns an ActiveStorage::Variant instance with the set of +transformations+ provided. This is only relevant for image
   # files, and it allows any image to be transformed for size, colors, and the like. Example:
   #
@@ -125,8 +127,20 @@ class ActiveStorage::Blob < ActiveRecord::Base
   #
   # This will create a URL for that specific blob with that specific variant, which the ActiveStorage::VariantsController
   # can then produce on-demand.
+  #
+  # Raises ActiveStorage::Blob::InvariableError if ImageMagick cannot transform the blob. To determine whether a blob is
+  # variable, call ActiveStorage::Blob#previewable?.
   def variant(transformations)
-    ActiveStorage::Variant.new(self, ActiveStorage::Variation.wrap(transformations))
+    if variable?
+      ActiveStorage::Variant.new(self, ActiveStorage::Variation.wrap(transformations))
+    else
+      raise InvariableError
+    end
+  end
+
+  # Returns true if ImageMagick can transform the blob (its content type is in +ActiveStorage.variable_content_types+).
+  def variable?
+    ActiveStorage.variable_content_types.include?(content_type)
   end
 
 
@@ -170,16 +184,16 @@ class ActiveStorage::Blob < ActiveRecord::Base
     case
     when previewable?
       preview transformations
-    when image?
+    when variable?
       variant transformations
     else
       raise UnrepresentableError
     end
   end
 
-  # Returns true if the blob is an image or is previewable.
+  # Returns true if the blob is variable or is previewable.
   def representable?
-    image? || previewable?
+    variable? || previewable?
   end
 
 
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